Embedded Systems and Software

Transcription

1 Embedded Systems and Software Lecture 12 Some Hardware Considerations Hardware Considerations Slide 1

2 Logic States Digital signals may be in one of three states State 1: High, or 1. Using positive logic polarity, this corresponds to a voltage level closer to the power supply than to ground. Example. For an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input voltage larger than 0.6 Vcc = 3 V is considered high, or logic 1. State 2: Low, or 0. Using positive logic polarity, this corresponds to a voltage level closer to ground than to the power supply. For example, for an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input voltage larger than 0.6 Vcc = 3 V is considered logic high, or 1. Example. For an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input less than 0.3 Vcc = 1.5 V is treated as a logic low or 0 In negative logic polarity, voltages levels closes to the supply than to ground are 0, and voltage levels closer to ground than to the power supply are 1. This is used in some serial communication protocols. State 3: Tristate or high-z, or simply Z. In this case the digital line is in a high-impedance state or floating. External components, often a pull-up resistor can pull the bus high or low. Hardware Considerations Slide 2

3 Driving an LED V cc = 5 V RR llllllllll AVR PB2 Use the LED s V-I characteristics from its datasheet to determine the forward voltage for a given current. Then apply Ohm s Law to determine RR llllllllll Hardware Considerations Slide 3

4 Driving a High Power Load Solid State Relay (SSR) SIP SSR Ratings of 660 VAC SCR output for heavy industrial loads AC or DC control Zero-crossing (resistive loads) or random-fire (inductive loads) output Cost ~ $14 Load Control Hardware Considerations Slide 4

5 Driving a High-Power External Load V cc = 5 V Depending on the SSR, one may need an external resistor here to limit current out of microcontroller port AVR PB2 R limit SSR Control ma 300 Ω Opto-isolated Solid State Relay 220 V, 5 A Load Hardware Considerations Slide 5

6 Driving a High-Power External Load V cc = 5 V AVR PB2 R limit V on = ma Opto-isolated Solid State Relay 220 V, 5 A Load Example: The absolute max rating for I/O current is 40 ma, and the SSR has V on = ma. Determine R limit Hardware Considerations Slide 6

7 Well-defined Reset Pin It is important that the RESET pin has a well-defined voltage in normal operation, otherwise it may randomly reset. Also, if a mechnical switch is used for reset, consider contact bounce As a minimum, pull the line high and add a capacitor to filter out any glitches that may get to the RESET line Hardware Considerations Slide 7

8 Well-defined Reset Pin This supervisor chip makes sure the MCUs RESET line is well-behaved when the RST button is pushed and released Reset circuitry for 28-pin development board Hardware Considerations Slide 8

9 Power Supplies Bridge Rectifier Voltage Regulator Hardware Considerations Slide 9

10 Power Supplies Power supply section from 28-pin development board Bridge rectifier converts ac to dc. Also allows us to use dc. Hardware Considerations Slide 10

11 Power Supplies Power supply section from 28-pin development board Smoothing Capacitor Hardware Considerations Slide 11

12 Power Supplies Power supply section from 28-pin development board Three Terminal Regulator Hardware Considerations Slide 12

13 Power Supplies Power supply section from 28-pin development board Sets Output Voltage Hardware Considerations Slide 13

14 Power Supplies Power supply section from 28-pin development board Jump for 3.3V Hardware Considerations Slide 14

15 Power Supplies Power supply section from 28-pin development board Needed for Stability Hardware Considerations Slide 15

16 Power Supplies Reverse-Polarity Protection Bridge provides reversepolarity protection What voltage is supplied to embedded system if the input voltage is 5 V? Assume Si diodes. Diodes should be Schottky rather than regular Si diodes. Why? Hardware Considerations Slide 16

17 Power Supplies Linear Three-Terminal Regulator Varying input voltage, say 7-14 V. Must be larger than output voltage + some headroom, typically 1-2 V Constant voltage: 3 V, 5 V, etc. Note that linear regulators cannot step up a voltage, but regulate down. Hardware Considerations Slide 17

18 Power Supplies Linear Three-Terminal Regulator LM78XX Three-terminal regulator pinout in TO220 package Heat sink to cool down regulator Hardware Considerations Slide 18

19 Power Supplies Linear Three-Terminal Regulator These capacitors are easily-overlooked but a crucial for proper operation Hardware Considerations Slide 19

20 Power Supplies Linear Three-Terminal Regulator Quiescent current for LM78xx regulators are large, and can waste lots of energy not suitable for batteryoperated equipment. Hardware Considerations Slide 20

21 Power Supplies MAX604 Linear Regulator Quiescent current for MAX604 a few micro-amps, thus much more suitable for battery-operated equipment. Hardware Considerations Slide 21

22 Power Supplies MAX724 DC/DC switching Regulator Very wide input range, efficient conversion to output voltage Note that switching regulator can step up voltages Hardware Considerations Slide 22

23 Decoupling Capacitors Consider a microntroller that runs on a 10 MHz clock. This means the clock period is 100 ns. The rise and fall time of the clock is thus on the order of 10 ns or shorter. The microcontroller s internal electronics (logic gates) create significant switching noise that appears on the power supply lines. The current is drawn in very short spikes on the clock edges, and if I/O lines are switching, the spikes will be even higher. This kind of current spike cannot be delivered over long power supply lines; the main source is (or should be) the decoupling capacitor. Decoupling capacitor ~ 0.1 μf The decoupling capacitor s (local) energy provides local storage to supply to switching circuits. Used properly, a decoupling capacitor can greatly reduces switching noise. Sometimes called a bypass capacitor Hardware Considerations Slide 23

24 Power Supply Decoupling Wrong way to decouple power supply Decoupling capacitor is ~ 0.1 μf Note: high loop current flows through the ground and can contaminate other circuits Hardware Considerations Slide 24

25 Power Supply Decoupling Better way to decouple power supply Inductor blocks residual current spikes from entering power supply. Typical value is 47 nh. Decoupling capacitor is very close to the controller, across its GND and Vcc pins. Large current spikes never flow through the circuit ground. Hardware Considerations Slide 25

26 External Clock Sources For crystals with frequencies larger than 400 khz For crystals with frequencies less than 400 khz, typically kHz watch crystal. Not supported on all AVRs Hardware Considerations Slide 26

27 External Clock Sources Crystals For simplicity, the oscillator capacitors are not always indicated, but they are crucial for proper operation, and in most cases the oscillator will not oscillate without these capacitors Capacitor values are ~ 20 pf, which are small. Hardware Considerations Slide 27

28 External Clock Sources Ceramic Resonators Connect to ucontroller 10 MHz ceramic resonator. Cost ~ $0.25 Equivalent circuit Lower Q than crystals. This means less stable, but also faster start-up time Are available with built-in capacitors Hardware Considerations Slide 28

29 External Clock Sources Question: What does ppm mean? Answer: Parts per million To convert from ppm to %, divide the number by 10,000. To convert from % to ppm, multiply by 10,000 Cost ~ $0.25 Cost ~ $0.75 Hardware Considerations Slide 29

30 Board Layout Hardware Considerations Slide 30

31 In normal operation, the 5 V linear regulator provides clean 5 V. D 1 drops V D(on). This is greater than (3 V + V D(on) ), so D 2 is reverse-biased and open. Without main power, D 2 is forward biased turn on, and powers the controller. Question: what type of diodes should D1, D2, be, and why? Answer: Schottky diodes, because they have lower turn-on voltages than Si diodes. Thus, these diodes dissipate less power. Hardware Considerations Slide 31

32 Recall Earlier Slide Example. Turning LEDs on and off Connecting LEDs Note LEDs will light up when ports pins are pulled LOW Data Direction Register Configuring port pins for output Turn LED at PB1 OFF sbi DDRB,1 ; PB1 is now output sbi DDRB,2 ; PB2 is now output sbi PORTB,1 ; LED at PB1 off Turn LED at PB1 ON cbi PORTB,2 ; LED at PB2 on Hardware Considerations Slide 32

33 DDRB, PORTB, PINB, I/O ports on ATmege88P: PORTB, PORTC, and PORTD Individual pins of the ports are PB1, PD4, etc. Complex circuitry sits behind each port pin. Pins can be digital input, digital output, and analog Input Three registers located in I/O space DDRB Data Direction Register for PORTB PORTB Digital Output Register for PORTB Simplified I/O equivalent schematic PINB Input Register for PORTB Hardware Considerations Slide 33

34 DDRB, PORTB, PINB, All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the SBI and CBI instructions. The pin driver is strong enough to drive LED displays directly. All port pins have individually selectable pull-up resistors with a supply-voltage invariant resistance. The same applies when changing drive value (if configured as output) or enabling/disabling of pullup resistors (if configured as input). Each output buffer has symmetrical drive characteristics with both high sink and source capability. All I/O pins have protection diodes to both VCC and Ground Pull-up resistor ensures that is at expected logic levels if external devices are disconnected. Protection Diodes The idea is that is that it weakly "pulls" the pin to Vcc. The resistor is high-resistance enough that, if something else strongly pulls the wire toward 0V, the pin will go to 0V. Hardware Considerations Slide 34

35 DDRB, PORTB, PINB, Configuring a PIN DDRB Data Direction Register for PORTB PORTB Digital Output Register for PORTB PINB Input Register for PORTB Hardware Considerations Slide 35

36 Unconnected Pins If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode and Idle mode). The simplest method to ensure a defined level of an unused pin, is to enable the internal pullup. In this case, the pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended to use an external pull-up or pull-down Connecting unused pins directly to VCC or GND is not recommended, since this may cause excessive currents if the pin is accidentally configured as an output. Hardware Considerations Slide 36

37 Example. Consider the table below, extracted from the DC Characteristics section of the ATtiny45/V microcontroller. Assume the power supply voltage is 2 V, and that PB0 is configured as digital input. Determine the threshold voltages for logic 1 and 0 on input. Solution. PB0 is not XTAL or RESET, so V IL and V IH apply. V I0 = 0.2Vcc = = 0.4 V, and V I1 = 0.7Vcc = = 1.4 V Hardware Considerations Slide 37

38 Pull-Ups Revisited The Pull-up Disable PUD bit in MCUCR disables the pull-up function for all pins in all ports when set. If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output pin. Self-Study Give a code snippet to configure PB1 on the ATmega88PA as an input with the internal pull-up resistor activated. Comment your code. Hardware Considerations Slide 38

39 Pull-Ups Revisited Pull-up resistors can have a significant spread between parts, and even individual pins on a port, so beware Hardware Considerations Slide 39

40 Microcontroller Fuses AVR ATtiny45 8-Pin Microcontroller Except for GND and VCC all other pins can perform at least 4 functions PB5 can be hardware RESET or an ADC input PB3 & PB4 can be ADC inputs or where crystal for external oscillator goes How does one configure controller for the external environment? Hardware Considerations Slide 40

41 Electrical Connections Test circuit, from the data sheet Pullup resistors Low-pass filter Low-pass filter Remember contact bounce? Hardware Considerations Slide 41

42 Switch Bounce and Debouncing Bounce Hardware Considerations Slide 42

43 Switch Bounce and Debouncing When the switch closes, the capacitor discharger through Rb Voltage Question: what is the fall time? Make sure you can calculate this. t Hardware Considerations Slide 43

44 Wired-Or Application: Bus Internal Pull-up Resistor OFF Bus is normally high Internal Pull-up Resistor OFF Any microcontroller can pull the bus low Hardware Considerations Slide 44

45 Reading a Simple (3 4) Keypad Note the weak pull-ups on PDB- PD7. These must be enabled in software Hardware Considerations Slide 45

46 Reading a Simple (3 4) Keypad 0 Step 1: Detecting a key press a) Drive PC4, PC5, PC6 low. b) READ Port D and look for zero on any of PD4-PD7 c) If so, a key has been pressed 0 0 Hardware Considerations Slide 46

47 Reading a Simple (3 4) Keypad Step 2: Debounce. Wait an appropriate period for the key switch to stabilize (10 ms is probably a good waiting time) Hardware Considerations Slide 47

48 Reading a Simple (3 4) Keypad 0 1 Step 3: Search for the pressed key a) Drive PC4 Low, PC5,PC6 High b) Read Port D PD4 = 0 1 pressed PD5 = 0 4 pressed PD6 = 0 7 pressed PD7 = 0 * pressed c) If pressed key is not found in column 1, repeat the process as necessary for the other two columns 1 Hardware Considerations Slide 48

49 Reading a Simple (3 4) Keypad 1 0 Step 4: Search for the pressed key a) Drive PC5 Low, PC4,PC6 High b) Read Port D PD4 = 0 2 pressed PD5 = 0 5 pressed PD6 = 0 8 pressed PD7 = 0 0 pressed c) If pressed key is not found in column 2, repeat the process as necessary for the other two columns 1 Hardware Considerations Slide 49

50 Reading a Simple (3 4) Keypad 1 Step 5: Search for the pressed key a) Drive PC6 Low, PC4,PC5 High b) Read Port D PD4 = 0 3 pressed PD5 = 0 6 pressed PD6 = 0 9 pressed PD7 = 0 # pressed 1 0 Hardware Considerations Slide 50

51 Hardware Considerations Slide 51